1. Field of the Invention
This invention relates to a filter, and more particularly, to a filter (e.g., a cigarette filter) which is formed of an interpenetrating polymer network (IPN) made of at least two polymers which are substantially (readily) water soluble in their salt form and which ionically interact. The IPN is used to form fibers, which are in turn used to form a filter tow which, upon disposal following use, is sufficiently water soluble to break down under normal atmospheric conditions. This interaction makes the invention highly desirable for use in cigarette filters.
2. Description of the Related Art
Typical filters were initially composed of cellulose acetate (U.S. Pat. No. 2,917,054), cotton or paper fibers. Subsequent cross liking of celluloses (U.S. Pat. No. 4,350,173) include cellulose modified with isocyanates or high molecular weight fatty acids (U.S. Pat. No. 4,015,611), carboxylated cellulose (U.S. Pat. No. 3,589,364), and cellulose ethers or esters (U.S. Pat. Nos. 2,639,239; 3,723,413). Polyolefins have also been used (U.S. Pat. Nos. 2,780,228; 2,966,157; 3,144,025; 3,880,173; 4,261,373; 4,729,391; 5,063,945), as well as polyolefin copolymers (U.S. Pat. Nos. 3,393,120; 4,579,130), and polyesters of terephthalic acid and derivatives (U.S. Pat. No. 4,188,960).
The above processes generally use an external cross linker and solvent to achieve insolubility of the final fiber in water (the startup polymer generally also is not water soluble either). This is a drawback.
Bonding between the fibers of a filter is important to maximize the filtering capacity of the filter by providing a sufficiently tortuous flow path. Bonding is particularly important in filters where the polymers are made by extrusion or other techniques that result in a parallel orientation of the fibers, thus providing a very easy flow path for the smoke. Bonding also provides the filter with strength which allows easier manufacture and prevents the filter from disintegrating during use. Yet, the flow path through the filter cannot be so tortuous as to prevent the smoker from drawing air through the cigarette. Thus, the appropriate balance between filtration capacity and pressure drop must be maintained.
Examples of this kind of bonding between fibers, particularly cellulosic fibers, has been achieved with the use of plasticizers such as triacetin mixed with a dialkyl orthophthalate (U.S. Pat. No. 3,190,295), alkylene glycol and polyalkylene glycol esters of acetoacetic acid (U.S. Pat. No. 3,227,164), esterified improved plasticizer type hardening in phenol removing agents such as the esterified propionic acid esters of tri-, tetra-, and pentaethylene glycol (U.S. Pat. No. 3,393,684), polyethylene glycol diacetate (U.S. Pat. No. 3,229,699), glycerol triacetate and polyethylene glycol diacetate (U.S. Pat. No. 3,640,742).
Another method of bonding fibers is to use a mix of polymers, one of which is thermofusible. Thus, on heating the filter, one of the polymers melts creating the desired bonds (U.S. Pat. Nos. 4,261,373; 4,379,465; 4,579,130). Other methods of bonding fibers into nonwoven articles are described (U.S. Pat. Nos. 3,734,841; 2,811,029; 2,978,785; 3,102,835; 3,271,220; 3,365,354; 3,573,130).
Crimping of the fibers also contributes to a tortuous flow path and is often performed on filters to be used for cigarettes. Crimped cellulose acetate fibers may be bonded with nontacky essentially nonvolatile non-migrated organic plasticizers to increase rigidity (U.S. Pat. Nos. 3,003,504; 3,008,474). Crimping is also thought to be necessary for polyolefin fibers (U.S. Pat. No. 2,966,157).
Other methods of filter manufacture are known including meltblown nonwovens (U.S. Pat. Nos. 3,595,245; 5,053,066), folded or pleated polymer sheets (U.S. Pat. Nos. 3,346,682; 5,053,066), porous membranes (U.S. Pat. No. 5,019,262), and foamed cellulose (U.S. Pat. No. 4,282,890).
Other filter materials are also known such as kaolin fibers with a high alumina and silica content (U.S. Pat. No. 4,729,389). Other additives include fibers coated with additives such as organic compounds (U.S. Pat. No. 4,729,390), flour (U.S. Pat. No. 2,917,054), citric acid (U.S. Pat. Nos. 3,424,173; 3,424,172; 5,150,721), glycerin and tobacco extract (U.S. Pat. No. 5,076,295), flavor additives (U.S. Pat. No. 3,144,024) and nicotine (U.S. Pat. Nos. 3,280,823; 4,736,755).
Cellulosic polymers in filters have been at least partially replaced with polyolefin polymers. Specific polyolefins employed include polypropylene, polyethylene (U.S. Pat. Nos. 3,393,120; 4,379,465; 5,025,815), and polybutadiene, poly-1-butene, polyisobutylene, polyisoprene, poly-4-methyl-1-pentene or combinations thereof (U.S. Pat. No. 4,736,755). Polypropylene has also been mixed with thermofusible fibers such as polybutylene, polyethylene or copolymers or tripolymers thereof (U.S. Pat. No. 4,579,130).
Cellulose acetate filters generally require the use of relatively costly plasticizers such as triacetin to provide the desirable bonding between the fibers of the filter. Additional costs are incurred to handle and dispose the hazardous solvent, remove the solvent from the final product and dispose of waste solvent. The use of organic solvents in the manufacturing of cigarette filters present other problems as well. Trace organic solvents, left on the cigarette filter present health concerns, especially in light of the cigarette filter being held in the user's mouth with hot gases being drawn through the filter. Additionally, the use and disposal of organic solvents and the Volative Organic Compounds (VOCs) which can be created from their use, present environmental concerns relating to environmental contamination.
Crimping increases the bulk of the filter materials and the filtration capacity of a filter. However, it requires an additional manufacturing step and also decreases the strength of the filter fibers. This loss of strength can lead to increased difficulty and expense in shaping the filters to a desired form.
Although polyolefins have certain advantages over cellulose tows, they are not susceptible to bonding with conventional high boiling plasticizers. Instead, thermofusible polymers generally must be mixed with the polyolefin. Filters of this type must be subjected to an expensive heating step to create the desired bonds. Polyolefins can also be bonded by mixing with a plasticizer sensitive polymer. However, this leads to the use of expensive plasticizers as discussed above.
Furthermore, polyolefins generally have very smooth surfaces that decrease the filtration capacity of the fiber. Fibrillation and similar techniques have been used to increase the surface area, tortuousness of flow path and filtration capability of polyolefin filters. Unfortunately, these techniques are also time consuming, expensive and may weaken the fibers.
The known fibers discussed above, and particularly the polyolefin filters, suffer from the significant shortcoming that they do not biodegrade after the cigarette is smoked and the filter is discarded. Thus a used cigarette filter discarded on the ground remains an eyesore and environmental problem. Various other drawbacks also exist.